There is a considerable need for molecular precursors for Chemical Vapour Deposition (CVD) or Sol-gel processes to fabricate Lanthanide based materials. These are of potential importance as precursors of multimetallic oxides. For such precursors to find ready application, they should be stable in the gas phase and have good mass transport properties, thereby allowing the formation of a thin or thick film of either the metal or metal oxide to be deposited onto the desired substrate.
A major potential use of such precursors is in the synthesis of electroceramics, e.g. high temperature superconductor ceramic thin or thick films for use in electronic devices such as YBa.sub.2 Cu.sub.3 O.sub.7-x ; see P. P. Edwards et. al. Chemistry Britain, 1987, 23-26., Pb.sub.2 Sr.sub.2 LnCu.sub.3 O.sub.8-x ; M. O'Keefe and S. Hansen, J. Am. Chem. Soc. 1988, 110 1506, R. J. Cava et. al. Nature, (London), 1988, 336, 211-214; La.sub.2-x Sr.sub.x CuO.sub.4 ; Bednorz and Muller, Z. Phys. B. Cond. Matter, 1986, 64, 189-195; piezoelectrics such as LaCuO.sub.2 ; Muller-Buschbaum, Angew. Chem. 1989, 28, 1472-74 and phosphors and fuel cells.
Multimetallic oxide based ceramics are conventionally made by "heat and bake" technology; see D. Segal, Chemical Synthesis of Advanced Ceramic Materials, Cambridge University Press, Cambridge, 1991. This approach relies upon the intimate mixing of metal-oxygen based materials (e.g. metal carbonates, nitrates or hydroxides) by the use of techniques such as ball-milling, fusion processes, and uniaxial or hot isostatic pressing. Although these processes are attractive owing to their inherent simplicity and low cost, there are several inherent disadvantages. These are high temperature processing and post-annealing under a flow of oxygen gas, which ensures that any meta-stable phases cannot be accessed by this approach. There is also the added difficulty of phase inhomogenity, e.g. tetragonal and orthorhomic forms of YCu.sub.2 Ba.sub.3 O.sub.7-x present in the same material, and also the presence of ionic impurities (e.g. BaCO.sub.3) frequently found at the grain boundaries.
An alternative strategy involves the use of metal alkoxides or .beta.-diketonates; see Mehrotra et. al. Chem. Rev. 1991, 91, 1287-1302. These compounds are readily obtained as crystalline solids of known stoichiometry, high purity, good solubility in organic solvents, and long term stability in an inert atmosphere, and are sufficiently reactive that most reactions occur at or near room temperature. By using such materials fine control of molecular stoichiometry is possible and access to previously unobtainable metastable phases is achievable. These materials find extensive use in either sol-gel or chemical vapour deposition (CVD) processes, which involve the formation of either thick films (sol-gel spun coating) or ultrathin films (20 .ANG. or less) for optical or microelectronic applications by CVD.
However, conventional lanthanide precursors for metal oxide films have several drawbacks, notably in the high residue left in commercial evaporators/bubblers for CVD and poor stability in the atmosphere. The use of fluorinated precursors results in the formation of LnF.sub.3 which has to be removed with either superheated water vapour or air at elevated temperature to yield the required oxide based film. Therefore, to produce epitaxial or high quality films it is important to avoid the use of fluoride based compounds, even though such complexes have excellent vapour pressure and mass transport properties. Thus, as stated above, a precursor is required that vapourises without any decompostion and remains intact in the vapour phase for considerable time periods, i.e. for at least the length of time of the CVD process.
Lanthanide metal alkoxides and .beta.-diketonates are well known materials; see K. S. Mazdiyasni et al Inorg. Chem., 1966, 3, 342-347; K. S. Mazdiyasni et al J. Less-Common Met., 1973, 30, 105-112, and R. C. Mehrotra et al, Metal Beta-diketones", Academic Press, London, 1978. A considerable degree of diversity has been previously found in their chemistry, notably with added Lewis bases; see T. Moeller et.al. Gmelin Handbook of Inorganic Chemistry, Sc, Y, La-Lu Rare Earth Elements Part D3, 8th Edn, Springer, Berlin, 1981. The most commonly used strategies for preparing volatile metal precursors are the use of bulky or alternatively fluorinated ligands that encapsulate the metal ions, and thus create discrete molecular species. This phenomenon occurs due to reduced intermolecular associations between metal centres, and therefore changes the orientation of the packing in the solid or liquid states; this in turn gives rise to enhanced thermal and mass transport properties. This approach has been recently adopted for the lanthanide complexes; see W. J. Evans et al., Inorg. Chem., 1989, 28, 4308-4314; M. J. McGeary et al., Inorg. Chem., 1991, 30, 1723-1724; E. H. Barash et al., Inorg. Chem., 1993, 32, 497-502; and R. E. Sievers, Science, 1978, 201, 217-223.
Because of their large ionic radii and coordination numbers the lanthanides are difficult to coordinatively saturate to yield monomeric complexes. Bulky ligands are limited in their ability to coordinatively saturate these highly Lewis acidic metals, i.e. poly-functionalised ligands have been extensively used, Y(OCH.sub.2 CH.sub.2 OMe).sub.3 !.sub.10 ; see O. Poncolet et al. J. Chem. Soc., Chem. Commun., 1989, 1846-47; and Y.sub.3 (OCH.sub.2 CH.sub.2 OMe).sub.5 (acac).sub.4 ; see O. Poncolet et.al. Inorg. Chem., 1990, 29, 2885-2890. There are also other lanthanide complexes which contain simple ligands (e.g. Pr.sup.i O and Bu.sup.t O), which are not sufficently electron rich to supply the electronic and steric requirements of these metals; see D. C. Bradley et al., Polyhedron, 1990, 9, 719-725 and 10, 1049-1056. This can lead to highly associated or indeed polymeric complexes, where the ligand is MeO.sup.- or EtO.sup.-. To date, there has been little success at controlling the degree of oligomerisation of lanthanide molecular precursors.
There are a number of synthetic strategies which may be employed to prepare complexes. The most common route utilizes metathesis. A modification of this, especially where the pK.sub.a of the beta-diketone is too low (e.g. acac-H), is to use a water-ammonia mixture to drive the reaction to completion, e.g. see Ln(acac).sub.3 (H.sub.2 O)!, K. J. Eisentraut et al, J. Am. Chem. Soc., 1965, 87, 5254-5259; and G. S. Hammond et al, Inorg. Chem., 1963, 2, 73-75. See also T. Moeller et al. Gmelin Handbook of Inorganic Chemistry, Sc, Y, La-Lu Rare Earth Elements Part D3, 8th Edn, Springer, Berlin, 1981.
These known precursor syntheses show limited systematic control of aggregate size and give poorly characterised materials having poor moisture and thermal stability, and a short shelf life. It is therefore highly desirable to sythesise thermally stable and highly soluble materials which are suitable for either CVD or Sol-gel applications.
Compounds for this purpose must satisfy the following physical and chemical criteria:
defined identity and purity. PA1 air and moisture stability for ease of handling. PA1 low melting point for use in conventional CVD bubbler source chambers. PA1 good solubility in a wide range of organic solvents. PA1 significant volatility at low temperature. PA1 clean pyrolysis at substrate temperatures. PA1 give deposited layers free of unwanted impurities.
The present invention provides compounds meeting at least some of these criteria.
The compounds of the invention can be used as precursors for deposition of oxide layers by the chemical vapour deposition (CVD) technique. Rare earth oxides are employed alone or in conjunction with other metal oxides as ceramic or glass layers in a range of advanced materials such as superconductors, piezoelectrics, fuel cells, optoelectronics, radiation detectors, catalysts and to provide thermal and abrasion resistance. The compounds can be used in making devices for use in information technology, medical instrumentation and energy conservation.
The compounds of the invention are the rare earth compounds of formula: EQU (ML.sub.3).sub.x A!.sub.y
where M represents one or more metals chosen from the rare earth metals and yttrium, L is a bidentate ligand, A is a polyether, polyamine or polyether-amine, and x and y are each 1 or 2 but are not both 2. They may be regarded as monomers of formula ML.sub.3 A or bridged dimers of formula (MI.sub.3).sub.2 A or (ML.sub.3 A).sub.2.
The bidentate ligand L may be a .beta.-diketonate anion containing a group of formula: ##STR1## derived, more especially, from a compound of formula: EQU R.sup.i R.sup.ii R.sup.iii CCOCHR.sup.iv COR.sup.v
where R.sup.i, R.sup.ii, R.sup.iii, R.sup.iv and R.sup.v are each hydrogen, alkyl of 1 to 6 carbon atoms optionally substituted by fluorine or phenyl, or fluorine, and R.sup.v may also be alkyloxy of 1 to 6 carbon atoms optionally substituted by fluorine, amino, alkylamino, or dialkylamino in which each alkyl has 1 to 6 carbon atoms optionally substituted by fluorine.
Preferably the ligand L is derived from a .beta.-diketone, especially from one or more of acetylacetone, tetramethylheptanedione, trifluoroacetylacetone, hexafluoroacetylacetone, and 1,5-diphenylpentanedione.
The polyether, polyamine or polyether-amine A may be represented by the formula: EQU R.sup.i --Y(CR.sup.ii R.sup.iii CR.sup.iv R.sup.v --Y).sub.n R.sup.vi
where each of R.sup.i, R.sup.ii, R.sup.iii, R.sup.iv, R.sup.v and R.sup.vi is hydrogen or alkyl of 1 to 6 carbon atoms, Y is --O--, --NR.sup.vii (where R.sup.vii is hydrogen or alkyl of 1 to 6 carbon atoms), or a mixture thereof, and n is 1 to 10.
Preferred polyethers may be represented by the formula: ##STR2## wherein R.sup.1, R.sup.2 and R.sup.3 are each hydrogen or alkyl of 1 to 4 carbon atoms and n is 1 to 10, and preferred polyamines by the formula: ##STR3## wherein R.sup.1, R.sup.2 and R.sup.3 and n are as hereinbefore defined
Especially preferred polyethers are those wherein R.sup.1 and R.sup.2 are each alkyl of 1 to 4 carbon atoms, R.sup.3 is hydrogen, and n is an integer from 1 to 7, more particularly monoglyme, diglyme, triglyme, tetraglyme, and/or heptaglyme.
Especially preferred polyamines are those wherein R.sup.1 and R.sup.2 are each alkyl of 1 to 4 carbon atoms, R.sup.3 is hydrogen, and n is an integer from 1 to 3, more particularly tmeda, pmdeta, and/or hmteta.
When A is derived from monoglyme, tetraglyme, tmeda, or pmedta, the monomeric structure is generally formed, i.e. ML.sub.3 A. When A is triglyme or heptaglyme a bridged dimeric compound is usually obtained, i.e. (ML.sub.3).sub.2 A, and with diglyme, a dimeric structure having two bridging diglyme molecules can be obtained, i.e. (ML.sub.3 A).sub.2.
Type I structure x=1, y=1
An example of this type of monomeric structure is shown by La(thd).sub.3 (tetraglyme)!. The lanthanum atom binds to all three bidentate thd beta-diketone ligands and to only three of the five possible oxygen atoms of the tetraglyme ligand. Thus the lanthanum atom prefers to adopt a nine-coordinate rather than a possible eleven-coordinate site. A square anti-prismatic geometry is observed for this complex, with the ninth coordinating oxygen atom O(5)! capping one of the square faces. In this complex the coordinated portion of the glyme has one short La-O bond, 2.706(7) .ANG. O(2)!, and two long La-O bonds, 2.781(6) O(5)! and 2.751(7) .ANG. O(8)!. The most noticeable and clearly unusual feature of this complex is the presence of the uncoordinated portion of the tetraglyme chain C(9)-C(15) which, with the exception of the terminal methoxy bond, is approximately planar. A study of a space-filling model of this complex indicates that the coordination of an additional glyme oxygen centre is not favourable because of the presence of the three tightly held thd beta-diketone ligands and with three of the five available glyme oxygen atoms saturating the lanathanum metal centre.
Type II structure x=2, y=1
An example of this type of structure is shown by {Eu(thd).sub.3 }.sub.2 (triglyme)! which consists of two Eu(thd).sub.3 moieties linked together by a triglyme ligand involving a unique bonding orientation. Both metal atoms are eight coordinate with the overall coordination polyhedron being distorted square antiprismatic. One of the most interesting features of this complex is the observation that the triglyme can act as both a chelate and also as a bridging ligand via the central ethylene bridge C(37)-C(38)!. This structure allows the utilisation of all four potential binding sites of the glyme ligand.
The coordination numbers of the metals in these complexes are eight or nine which is believed to be the principal reason for the advantageous chemical and physical properties of the compounds (see Table 1 below). Thermal behaviour has been studied by thermogravimetric analysis and clearly reveals that these materials volatilize into the gaseous phase intact, and the first derivative peak shows essentially 96.+-.2% sublimation for these compounds. The related melting behaviour has been examined by the use of differential scanning calorimetry which has demonstrated a marked reduction in melting point from 200.degree.-250.degree. C. to 60.degree.-130.degree. C. on the addition of the multidentate Lewis base ligand.
The compounds of the invention have an advantageously low molecular weight in relation to the amount of rare earth metal or yttrium present. Thus the average molecular weight per metal atom is usual below 2000 and preferably below 1500, and in especially advantageous cases can be below 1000.
The following Table gives examplary physical properties of some compounds of the invention.
TABLE 1 __________________________________________________________________________ evaporation temp. .degree.C./ T.sub.50% / residue pyrolysis Compound m.p./.degree.C. 10.sup.-2 mm Hg. .degree.C. % temp. .degree.C. __________________________________________________________________________ La(thd).sub.3 (tetraglyme)!.sup.a 59.9 110 287 3.2 312 Eu(thd).sub.3 !.sub.2 (triglyme) 128.2 115 258 3.7 287 Tb(thd).sub.3 !.sub.2 (triglyme) 115.5 120 242 4.5 330 Y(thd).sub.3 !.sub.2 (triglyme) 95.4 100 223 2.6 255 Pr(thd).sub.3 (triglyme)! 122 140 248 2.3 385 EuY(thd).sub.6 !(triglyme) 113.8 105 246 5.1 272 Tb(thd).sub.3 (diglyme)!.sub.2 64.3 110 257 2.2 288 Y(thd).sub.3 !.sub.2 (hmteta) 123.1 120 251 1.9 282 LaTm(thd).sub.6 !(triglyme) 55.9 115 246 4.9 286 __________________________________________________________________________
a. This complex is the only material observed to lose its polydentate ligand in the vapour phase to yield La.sub.2 (thd).sub.6 ! at ca. 180.degree. C.
The compounds of the invention have excellent solubility in a wide range of organic solvents, e.g. aliphatic solvents, such as n-pentane, hexane, and heptane; aromatic solvents such as benzene, toluene and xylene and coordinating solvents, e.g. diethylether, tetrahydrofuran, di-n-butylether, dimethylsulphoxide, acetonitrile, pyridine, and chloroform. Indeed, the outstanding solubility shown by the majority of the new compounds ensures that they do not crystallise out of organic solvents. If desired in a crystalline form, then all the organic solvent must be removed, and the compound is crystallised from the oily material remaining.
The excellent solubility of the new compounds in organic solvents makes them suitable for use as additives in lubricants and fuels, including fuels for internal combustion engines and, more especially, hydrocarbon fuels for compression ignition (diesel) engines.
The compounds stated in the prior art to have formulae such as "Ln(thd).sub.3 " where Ln is any rare earth metal or yttrium, actually contain coordinated water or other adducted ligands. Unadducted molecules are oligomers with melting points in the range 200.degree.-260.degree. C. with evaporation commencing at a slightly higher temperature. Our X-ray studies have shown that these compounds are dimeric complexes, e.g. Gd.sub.2 (thd).sub.6 !, with two bridging thd ligands. Unless strictly anhydrous conditions are maintained during preparation they are heavily contaminated with hydrated species which behave unpredictably because of intramolecular hydrolysis when the compounds are heated to their vacuum evaporation points. In general the rate of evaporation of the prior art compounds declines with time and source chambers become clogged with residue.
These complexes have infrared absorption bands at 1609.+-.5, 1586.+-.5 and 1540.+-.5 cm.sup.-1, assigned to .nu.(C---O) stretching modes and bands at 1575.+-.2 and 1500.+-.5 cm.sup.-1 assigned to the .nu.(C---C) stretching modes. In the compounds of the invention the glyme ligand .nu.(C---O) bands were observed in the region 1130.+-.10 cm.sup.31 1, a shift of ca. 50 cm.sup.-1 compared with the prior art compounds containing no glyme ligand, indicative of strong M--O bonding.
A reason for the oligomerisation and hydration of the prior art compounds is the lack of controlled saturation of the coordination sphere of the metal ion. The compounds of the invention have linear polyether molecules whose oxygen atoms act as Lewis base donors to saturate the Lanthanide metal ion coordination sphere. The outermost architecture of the adducted molecule comprises hydrocarbon groups whose neighbour interactions are weak Van der Waals attractions. Consequently the molecules of this invention exhibit little tendency to associate, and have melting and evaporation points 80.degree.-150.degree. C. below those of the prior art compounds. Moreover, they do not pick up water upon air exposure.
Additionally, the thermal stability of the compounds of the invention in the vapour phase assists deposition by providing an activated species prior to the final step of pyrolysis to the metal oxide. This is another advantage of the compounds of the invention, see FIGS. 9-15. The majority of the complexes studied exhibit a sharp reversible melting point in their DSC spectra. The TGA curve shapes for these compounds reveal the presence of a single isothermal step, and near complete vapourisation of these materials by ca. 300.degree. C.
The absence of water in the compounds of the invention is of especial importance. Anhydrous synthesis route I below employs either metal amide or alkoxide starting materials dissolved in hydrocarbon solvent and gives excellent yields. However, these materials are expensive, and routes II or III below give almost as good yields starting from cheaper hydrated salts dissolved, e.g., in methanol. The presence of a small excess of the polyether or amine is apparently sufficient to expel adducted water from the metal coordination sphere.
According to a feature of the invention, the new compounds are made by reacting a rare earth compound of formula: EQU M(NR.sup.4.sub.2).sub.3 or M(OR.sup.5).sub.3
where M represents one or more metals chosen from the rare earth metals and yttrium, R.sup.4 is alkyl of 1 to 4 carbon atoms or trimethylsilyl, and R.sup.5 is alkyl of 1 to 4 carbon atoms optionally substituted by alkoxy of 1 to 4 carbon atoms with a bidentate ligand LH and a polyether, polyamine or polyether amine. The reaction may, more particularly, be carried out in a hydrocarbon solvent.
The reaction may be represented: EQU xy M(NR.sup.2).sub.3 +3xy LH+y A.fwdarw. (ML.sub.3).sub.x A!.sub.y +3xy R.sup.2 NH
Where R=Et, Pr.sup.i or SiMe.sub.3 are the preferred ligands, or EQU xy M(OR).sub.3 +3xy LH+y A.fwdarw. (ML.sub.3).sub.x A!.sub.y +3xy ROH
Where R=Pr.sup.i or Bu.sup.t are the preferred ligands, but may also be, e.g. Me, Et, Pr.sup.n, or MeOCH.sub.2 CH.sub.2 --.
According to another feature of the invention, the new compounds are made by reacting a rare earth compound of formula: MZ.sub.3 (H.sub.2 O).sub.6 where M represents one or more metals chosen from the rare earth metals and yttrium and Z represents an anion with an alkali metal derivative of the bidentate ligand LH and a polyether, polyamine, or polyetheramine A. The reaction may be carried out in an alcohol solvent using a halide, carboxylate, sulphate or nitrate of the rare earth metal.
The reaction may be represented: EQU xy MZ.sub.3 (H.sub.2 O).sub.6 +xy LNa+y A.fwdarw. (ML.sub.3).sub.x A!.sub.y +3xyNaZ+6xyH.sub.2 O
where M, L, A, x and y are as defined above and Z=halide, carboxylate, nitrate, or sulphate.
According to yet another feature of the invention, the new compounds are made by reacting a rare earth oxide, hydroxide or carbonate with the bidentate ligand LH and a polyether, polyamine or polyether-amine A. The reaction may be carried out in an organic, e.g. hydrocarbon, or aqueous solvent. It may be represented: EQU xy M.sub.2 O.sub.3 +xy LH+y A.fwdarw. (ML.sub.3).sub.x A!.sub.y +3xy H.sub.2 O
where M, L, A, x and y are as defined above.
In the compounds of the invention the metal centres are coordinatively saturated with the combined use of both a chelating type of Lewis base ligand, i.e. a glyme or amine, and a chelating bidentate group, e.g. a diketone. This presumably gives rise to the exceptional stability in the atmosphere of the new compounds, since the chelating ligands are less readily hydrolysed than monodentate ligands. Secondly, the use of multidentate ligands reduces the possibility of interactions between monomeric units. Third, the use of a preformed metal beta-diketonate (whether anhydrous or as a hydrate) leads to water free products. Thus anhydrous metal diketone derivatives can be prepared by a low cost route, e.g. by the use of simple hydrated complexes prepared via metathesis in alcohol/aqueous media. The observation that water can be removed from the hydrated starting materials is important, and only one equivalent of glyme ligand is needed and not an excess.
The compounds show a further novel property in several cases, e.g. La(thd).sub.3 (tetraglyme)! (see FIG. 1), Gd(thd).sub.3 !.sub.2 (tetraglyme) (see FIG. 7), Y(thd).sub.3 !.sub.2 (hmteta) (see FIG. 3), and Gd(thd).sub.3 !.sub.2 (heptaglyme) (see FIG. 6). In these complexes a portion of the multidentate ether or amine chain is not coordinated to the lanthanide metal centre and may be used to react with an incoming metal complex (e.g. a transition metal or lanthanide) to synthesise previously inaccessible metal combinations, e.g. La--Cu, La--Cu.sub.2, La--Mn, Y--Zr, or Gd--Ce.
The following abbreviations are used herein:
__________________________________________________________________________ thd tetramethylheptanedionate Me.sub.3 CCOCHCOCMe.sub.3 acac acetylacetonate MeCOCHCOMe tfa trifluoroacetylacetonate F.sub.3 CCOCHCOMe hfa hexafluoroacetylacetonate F.sub.3 CCOCHCOCF.sub.3 dpp 1,5-diphenylpentanedionate PhCH.sub.2 COCHCOCH.sub.2 Ph monoglyme monoethyleneglycol dimethyl Me(OCH.sub.2 CH.sub.2)OMe (or dme) ether diglyme diethyleneglycol dimethyl Me(OCH.sub.2 CH.sub.2).sub.2 OMe ether triglyme triethyleneglycol dimethyl Me(OCH.sub.2 CH.sub.2).sub.3 OMe ether tetraglyme tetraethyleneglycoldimethyl Me(OCH.sub.2 CH.sub.2).sub.4 OMe ether heptaglyme heptaethyleneglycol dimethyl Me(OCH.sub.2 CH.sub.2).sub.7 OMe ether tmeda tetramethylethylenediamine Me.sub.2 NCH.sub.2 CH.sub.2 NMe.sub.2 pmdeta pentamethyldiethylenetriamine Me.sub.2 NCH.sub.2 CH.sub.2 N(Me)CH.sub.2 CH.sub.2 NMe.sub.2 hmteta hexamethyltriethylenetetramine Me.sub.2 NCH.sub.2 CH.sub.2 {N(Me)CH.sub.2 CH.sub.2 }.sub.2 NMe.sub.2 __________________________________________________________________________